High-strength Cr—Mo steel for use in a boiler or chemical reactor is used under high-temperature and high-pressure atmosphere, and thus is required to have the high-level creep characteristics (hereinafter also referred to as a “creep-rupture resistance”) in addition to the high strength and toughness. Particularly, in recent years, each equipment described above tends to be exposed under the higher-temperature and higher-pressure conditions for operation from the viewpoint of effective operation, and is increasingly required to have the improved creep characteristics.
Such conditions are also required of a weld metal formed in welding the high-strength Cr—Mo steel. The weld metal is also required to exhibit creep characteristics at high level in addition to the high-temperature resistance and toughness after stress relief (SR) annealing, and the high cracking resistance (SR cracking resistance) to SR annealing, which are important issues.
For this reason, various techniques for improving the high-temperature creep characteristics of the weld metal have been hitherto proposed.
For example, Patent Document 1 discloses that the weld with both the good creep-rupture resistance and other characteristics can be obtained by defining a steel composition, a weld material composition, and weld conditions in detail. In the technique, however, the supposed level of the creep-rupture resistance is 800 hours under about 240 MPa at 550° C., which is not sufficient. The SR annealing condition for one time is a maximum time of 26 hours, which is short (which is the condition that tends to make the creep-rupture resistance high). The SR annealing is performed twice, which disadvantageously needs complicated processes.
Another technique is proposed which achieves both the adequate creep-rupture resistance and various characteristics by taking into consideration the components of a solid wire and bonded flux, and welding conditions (the amount of input heat) (see, for example, Patent Documents 2 and 3). In these techniques, however, the supposed SR annealing conditions are 700° C. and 26 hours, and the holding time is very short, which does not mean that the good creep-rupture resistance is also surely obtained under a stricter SR annealing condition.
Patent Document 4 discloses a further technique that achieves both the adequate creep-rupture resistance and various characteristics by controlling the balance between components of a weld metal and main elements of a base metal. In the technique, however, the supposed level of the creep-rupture resistance is 900 hours under 538° C.×206 MPa, which is not sufficient. The above document fails to describe the holding time of the SR annealing. It is unclear whether the good creep-rupture characteristics can be achieved.
On the other hand, as a technique paying attention to characteristics other than the high-temperature creep characteristics, for example, there has been proposed such a technique as that disclosed in Patent Document 5. The technique improves the toughness and the resistance to temper brittleness by controlling the form of the carbide, but fails to describe the improvement of the creep-rupture characteristics.